1. Technical Field
The invention relates generally to the field of ink compositions. More specifically, the invention relates to radiation curable ink compositions comprising a dendritic or hyperbranched polyester acrylate component.
2. Description of the Related Art
Radiation curable inks, e.g. for wide and super-wide inkjet printing have been known for some time. Ink compositions, however, have substantial problems meeting the increased demand for high performance inkjet ink, especially for the ink with fast curing speed, adhesion to a broad range of substrates, good resistance, and good flexibility.
Radiation curable inks are generally composed of monomeric and oligomeric materials, pigments, initiators, and additives, and optionally, a small amount of solvent. Radiation curable inks are printed on numerous substrates, both rigid and flexible, e.g. polyvinyl chloride (PVC), polystyrene, polycarbonate, acrylonitrile-butadiene-styrene (ABS), polyester, polyolefins, and textile materials. The ink performance, e.g. adhesion, scratch and rub resistance, flexibility, hardness, etc. are highly dependent on the ink compositions, especially the properties of the monomeric and oligomeric materials used in the ink compositions. Some existing radiation curable ink compositions are found to have low adhesion, which results in premature peeling or flaking of ink film from substrates. In addition, some ink compositions show poor flexibility on flexible substrates when folded or wrinkled as evidenced by cracking. The limited ability of ink compositions to adhere to a variety of substrates is increasingly problematic as the demand for using ink on a greater variety of substrates increases.
Existing radiation curable inks are generally based on (meth)acrylates and formulated by using mono-functional (meth)acrylate monomers as diluents and difunctional, trifunctional, or higher functional (meth)acrylate monomers as cross-linking agents to ensure that inks are cured with a certain amount of UV energy. Additional descriptions of (meth)acrylates can be found in WO 99/29787.
In existing ink compositions, low viscosity tri-functional or higher functional monomers are generally used to formulate fast curing inks where the ink dot is rapidly dried or cured after ejection from the nozzles of printing heads and deposited on the substrates using a small amount of radiation energy. These higher functional monomers sacrifice flexibility and adhesion for a fast drying capability, which significantly limits the application of the ink. Thus, there is a continuing need for radiation curable inks that have good adhesion to multiple substrates and improved flexibility that is capable of withstanding cracking during application.
U.S. Pat. No. 7,365,105 discloses a high amount of low viscosity tri-functional or higher functional monomers that are used in ink compositions to achieve a fast curing ink. Less than 10% hyperbranched oligomers are used in the ink compositions due to their high viscosity.
EP1375569 discloses radiation-curable mixtures comprising highly-branched polyesters having acrylate terminal groups that form high viscosity paint or ink mixtures where the viscosity is between 100-1000 centipoise (cP).
US 2006/0014852 discloses a radiation curable ink containing a hyperbranched polymer as a photoinitiator to formulate radiation curable inks and coatings.